Difficulties are often encountered when loads are to be transported up or down steps or stairs, which in some cases may be steep, as well as on flat or at least generally flat surfaces. Such difficulties are not confined to transporting items or materials under such circumstances, but similar difficulties occur in regard to transporting a wheelchair for a handicapped person who wishes to negotiate steps or stairs. In an effort to deal with such a difficult situation, a step-negotiating vehicle has been developed, comprising a chassis structure of low height, which is adapted to fit under a wheelchair or to carry a load thereon. The chassis structure is provided with fixing means for securing the wheelchair in position thereon, and carries a support and drive means comprising tracks which are passed around suitable rollers and guides, with at least one of the rollers for each track being drivable by means of a motor. Such a track-laying, step-negotiating vehicle (see U.S. Pat. No. 4,401,178) is provided with a fixing means in the form of an attachment or adaptor which is so designed as to engage a wheelchair which is disposed on the top of the chassis structure, thereby to permit the wheelchair with its occupant to be transported in safety. In that vehicle, as mentioned above, the motor-driven tracks pass around a respective sliding guide assembly which is rigid along the line of the lower run of the tracks where they engage the ground, with the end portion of the guide arrangement being curved up at one end, where the track extends around a guide or drive roller. Each track may also be passed around a star-like roller member adapted to provide the necessary guiding and driving action. In the region of the upwardly curved end portion of the track guide means, the chassis structure carries transport rollers which are arranged to engage the ground in such a way as to lift the adjacent portion of the tracks away from the ground so that when the vehicle is travelling on a substantially flat surface, the vehicle is not driven by way of the motor-driven tracks, but simply rolls along on the transport wheels. In order for the vehicle to be operated in that situation, on an at least substantially horizontal surface, the rollers are fitted into the side of the chassis structure or have to be lowered on to the ground by means of foot rest members.
However, the step-negotiating,track-laying vehicles of the kind indicated above, which are intended in particular for carrying a wheelchair, must be of narrow track or gauge so that the chassis structure with the driven tracks can be fitted into position under the wheelchair, within the narrow space available between the wheels of the wheelchair structure. The narrow track or gauge of the vehicle means that it tends to suffer from a lack of lateral stability, which is obviously a very serious difficulty when the vehicle is to be used for transporting a handicapped person in a wheelchair. In addition, in order to negotiate steep steps or stairs, the length of the tracks must be relatively great in order to provide sufficient stability in a fore-and-aft direction, and in order to prevent the vehicle with the wheelchair and its occupant from tipping forward, for example when going down a steep flight of steps. However, the substantial track length means that the maneuverability of the vehicle on a substantially flat surface is considerably impaired. Difficulties are encountered in attempting to find a satisfactory compromise between a track length which is sufficiently short to permit easy maneuverability of the vehicle while at the same time ensuring suitable stability in the fore-and-aft direction. Furthermore, the transport or support rollers which can be fitted to the chassis structure in order for the vehicle to move on a generally horizontal surface are troublesome in use because they must be removed in order for the vehicle to negotiate stairs or steps. A construction whereby the rollers can be lowered into contact with the ground by means of foot members is fairly complicated and expensive, and causes problems when the steps or stairs to be negotiated comprise two or more flights which are separated as by a landing or a level section since, in negotiating such stairs, the rollers have to be moved into and out of their operative position at each landing or level section, in making the transition between the level section and the adjacent steps or stairs. The correct position for the rollers is generally monitored by electrical means, which constitutes a further complication and possible source of trouble.